It’s live, prepare to self-destruct in 3..2..1.. HTTP Public Key Pinning (HPKP) A new HTTP header that allows web host operators to instruct user agents to remember (“pin”) the hosts’ cryptographic identities over a period of time. During that time, user agents (UAs) will require that the host presents a certificate chain including at least one Subject Public Key Info structure whose fingerprint matches one of the pinned fingerprints for that host.